hw/arm/bcm2835_peripherals: Improve logging

hw/arm/bcm2835_peripherals: Improve logging

Various logging improvements as once:
- Use 0x prefix for hex numbers
- Display value written during write accesses
- Move some logs from GUEST_ERROR to UNIMP

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20190926173428.10713-3-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>